During the final stages of the fabrication of many types of semiconductor devices or "chips", a single semiconductor die is mounted within a sealed package. A series of leads extend out through the package for connection to an external circuit. Electrical contact is made between conductive pads formed on the face of the die and the external leads using very thin metal wires. One end of each of these wires is bonded to a conductive pad on the die and the other end is bonded to a lead. Generally, the initial components in the packaging process are the dies and the leadframe strip. Several individual leadframes are formed in each leadframe strip. Each leadframe supports a semiconductor die for packaging and provides the external leads for the finished chip. Each leadframe strip is moved as a unit through the various stages of the packaging process.
In a conventional packaging process, the dies are attached to mounting paddles on the leadframe strip. Then, during a wire bonding operation, the bond wires are connected to the conductive bond pads on the die and the lead fingers on the leadframe strip. The dies, bond wires and inner lead fingers are encapsulated and a trim and form operation separates the individual packages on the leadframe strip and bends the outer lead fingers into the proper lead configuration. The present invention is directed to the wire bonding operation. Wire bonding machines are well known in the art. Earlier wire bonding machines were manually operated by an operator viewing the die through a microscope and positioning the bonding tool over the bonding site. More recently, automated wire bonding machines include optical systems for sensing the location of the bond pads and lead fingers and automatically positioning the bonding tool over each bonding site. The Kulicke & Soffa Model 1488 Automatic Bonder wire bonding machine is representative of the automated wire bonding machines that are now commonly used in semiconductor manufacturing.
During a conventional wire bonding process, and using a conventional wire bonding machine such as the Kulicke & Soffa Model 1488, a single die attached to a leadframe on the leadframe strip is held between a clamp and a heat block. The heat block heats the die and the leadframe to a temperature of about 150.degree. C. to about 350.degree. C. A bonding tool mechanically presses the bond wire to a bond pad on the die and then to a bonding site on the appropriate lead finger. The bond wire is typically a fine gold or copper wire that is threaded through the bonding tool. The end of the wire is heated by an electrical discharge or hydrogen torch to a molten state to form a ball of molten metal on the end of the wire. The molten ball is pressed by the bonding tool against the bond pad on the heated die to mechanically bond the bond pad and the wire. The bonding tool is then moved over the bonding site on the corresponding lead finger and the wire is pressed against the lead finger to mechanically bond the lead finger and the wire. The bond wire is then tensioned and sheared. This process is repeated for each bond pad on the die.
An important step in the wire bonding operation is the indexing of the leadframe strip. Indexing refers to the process of moving the leadframe strip to position each die one at a time under the bonding tool so that the optical sensors can locate reference points on the die and properly align the bonding tool over the bond pads and bonding sites on the lead fingers. In a conventional wire bonding machine, each leadframe strip is removed from a magazine wherein several strips are stored and inserted into a rail type carrier. Thereafter, the leadframe strip is moved or indexed along the carrier for each successive die wire bonding operation. After all wire bonding operation cycles are completed, the leadframe strip is moved out of the carrier into a second magazine where it may be stored or transported for further processing. During each indexing step, the leadframe strip is engaged by the indexing mechanism and moved along the carrier. The leadframes, which are made of thin metal sheets, are sometimes damaged during these operations. It would be advantageous to index the bonding tool to each die on the leadframe strip rather than indexing the leadframe strip to the bonding tool. This would minimize the number of times each leadframe strip must be handled and thereby reduce the risk of damaging the leadframe strips. In addition, by moving the bonding tool rather than the leadframe strip, the time required to position each die under the bonding tool could be reduced and overall throughput in the packaging process increased.
The horizontal positioning systems used in conventional bonding machines, however, do not provide sufficient range of motion or speed to allow for bonding tool indexing. Conventional bonding tool positioning devices, commonly referred to as an X-Y table, have a limited range of motion, typically only about 2 inches along both the X and Y directions of travel. What is needed is a bonding tool positioning system that provides a faster and greater range of motion than conventional systems, at least in the X direction along the line of travel of the leadframe strips, while still maintaining the precision necessary to accurately align the bonding tool over the bond pads and lead finger bonding sites.